Both sustained and nonsustained ventricular tachycardias were reproducibly induced in dogs 3 to 5 days after ligation of the left anterior descending coronary artery. Isochronal maps of ventricular activation were constructed from close bipolar electrograms recorded from the entire epicardial surface and selected intramural sites by a computerized multiplexing technique. The electrophysiologic data were correlated with the anatomic characteristics of the infarction. The induced tachycardias were due to reentrant activation in the surviving epicardial layer overlying the infarction. Cooling or cryoablation was applied to localized epicardial sites along the reentrant circuit to reversibly or permanently interrupt reentrant activation. The reentrant circuit could be consistently interrupted when cooling or cryoablation was applied to the distal part of the common reentrant wave front proximal to the site of earliest reactivation. Localized cooling of the site of earliest reactivation usually failed to interrupt reentry because the common reentrant wave front reactivated other sites close to the original reactivation site. Before interruption of reentry, cooling resulted in characteristic changes in conduction of the reentrant wave front. The study (1) fulfills Mines' criteria that circus movement reentry is the mechanism of the induced rhythms in this canine experimental model and (2) identifies the critical site along the reentrant circuit at which cryothermal ablation (or surgical interruption) of reentrant activation could be successfully accomplished.